1. Field of the Invention
This invention relates to a reactive power compensator for compensating a reactive-current component in an alternating-voltage system such as is known from the prior art (L. Abraham and M. Hausler: Blindstromkompensation uber Halbleiterschalter oder Umrichter (Reactive-current compensation by means of semiconductor switches or frequency converters), VDE Symposium on electronics, Hannover, 1969, pages 100-114).
2. Description of the Prior Art
The increasing use of electric machines and plant having a higher reactive-current consumption, for example arc furnaces, asynchronous machines or phase-control systems' leads to increasing reactive power requirements on the consumer side of the power supply system. On the one hand, this results in additional loading on the supply system and, on the other hand, makes it more difficult to stabilize voltage fluctuations in the system.
In order to prevent these undesirable effects or at least to reduce them to a tolerable dimension, compensation of the reactive current is increasingly undertaken at its point of origin, that is to say at the load itself. The decentralized type of reactive-current compensation requires use of a plurality of reactive-power compensators which, apart from having as short as possible a recovery time, are especially simple and thus inexpensive to construct and, in addition, provide for continuous compensation of the inductive or capacitive reactive currents occurring.
The reactive-power compensators currently used are usually combinations of switched capacitor banks (TSC: thyristor-switched capacitor) and controllable inductances (TCR: thyristor-controlled reactance). The capacity of the capacitor bank is here dimensioned to the maximum inductive reactive power occurring at the load. Since the capacitor bank can be switched in or out only in steps, the reactive power range between the steps is covered by the controllable inductance (German Offenlegungsschrift No. 1,932,272). In this type of reactive power compensation, the expenditure for the capacitor bank to be installed increases with increasing reactive power of the consumer so that considerable compensator costs must be expected with a high reactive power.
In contrast, in the abovementioned VDE Symposium reference use of a reactive-current converter for compensating reactive power has been proposed which essentially consists of a current converter bridge circuit including current converter rectifiers with forced commutation. The direct-current output of the bridge is terminated by a smoothing choke. At the alternating-voltage input, capacitors are connected in parallel which protect the current converter rectifiers from overvoltages during the forced commutation at the alternating-voltage system encumbered with reactance.
The progress in time of the forced commutation is controlled in this reactive-current converter, designated as "type A", in such a manner that the frequency of the alternating-voltage system is used to generate rectangular blocks of current of alternating polarity, which lead the system voltage by about 90.degree. in the compensation of inductive reactive currents and lag by about 90.degree. in the compensation of capacitive reactive currents.
Although the operation of such a reactive-current converter is possible without the large and expensive capacitor banks needed with other compensation methods, this presents various problems. Firstly, the amplitude of the reactive current emitted or accepted by the current converter can be adjusted only through the magnitude of a difference angle .alpha. by which the phase shift between reactive current and system voltage deviates from the ideal value of 90.degree.. The result is that, apart from the desired reactive power, an additional active power of changing amplitude is produced. Secondly, the reactive current converter can change only discontinuously between the inductive and the capacitive operating range, that is to say with a phase jump of 180.degree. so that the control range has large gaps. Finally, the reactive current emitted and accepted in blocks of current has a considerable component of harmonics which must be suppressed by elaborate input filters in order to prevent interfering system reactions.